In UWB (Ultra Wide Band) radio, accurate ranging using the wideband characteristic is possible. For example, the IEEE802.15.4a standard discloses a four-point ranging method and a ranging method by measuring roundtrip time. Also, as a method of measuring the position of a tag, there is a method of measuring a tag position by combining the above ranging result and a technique of estimating the azimuth where the tag is present as the radio wave arrival direction, and, in principle, the tag position is obtained from the principle of polar coordinates.
As a literature encompassing arrival direction estimating techniques of radio waves, for example, there is Non-Patent Literature 1. As shown in Non-Patent Literature 1, there are a beam sweep, null sweep (e.g. Capon method) and a method (e.g. MUSIC method) of determining a correlation matrix or covariance matrix from signals obtained by array antennas and calculating an eigenvector using the determined matrix.
However, actual pulses of IR-UWB (Impulse Response-UWB) have a wider band than a narrowband signal or carrier wave supposed in Non-Patent Literature 1, its frequency characteristic has many errors between general array element systems, and, consequently, large error occurs if the above methods are used.
By contrast with this, as a radio arrival direction estimating apparatus using UWB, for example, Patent Literature 1 discloses an apparatus that has a plurality of distance measurement units and that estimates the radio wave arrival direction using the arrival time difference of radio waves from a tag.
FIG. 1 shows a configuration of array antenna 10 having reception units (antenna elements) 11 to 14. Array antenna 10 calculates the azimuth of an object using the difference of timing a signal reflected from the object arrives at each reception unit.